In the electric utility industry there are two items of equipment which are common to all electrical generating plants, to wit, steam boilers and turbine generators. Steam boilers are connected to turbine generators by piping means usually of great diameter, such as twenty-four inches or more. Irrespective of the various means of fitting these pipes together and to the boilers and turbines, such as by bolting, welding and the like, it is a matter of practical experience that workmen inadvertently tend to leave various objects in these large diameter pipes, such as hammers, wrenches, assorted other tools, hard hats, lunch boxes, bottles, buckets and the like. If the piping is welded, then there is the possibility that welding rod, welding flux, welding scale and various oxide scales naturally accumulated within the pipe will also be a source of contamination. Additionally, other forms of contamination, such as sand, dirt, rags, etc., must also be eradicated from the system.
Thus, before a power plant can start up to generate electricity, care must be taken to insure that every foreign substance within the pipes has been discharged before steam is permitted to be carried to the turbines. Otherwise, the delicately balanced and finely machined turbine blades would be seriously damaged by impingement of a mixture of super heated steam and foreign substances. It is therefore customary to install in the pipeline intermediate the boiler and the steam generator a valve, referred to as a blowdown valve, which can be quickly opened to effect a sudden pressure drop concurrent with high velocity escape of steam from the system which carries with it all of the foreign particles and objects trapped within the piping system. A target board is set up in line with the valve outlet to intercept flying objects being ejected through the valve and the valve is left open until no further foreign matter can be detected coming from the system. Whereupon, a value judgment must be made when the system is considered to be free of harmful debris and in condition for transmission of steam from the boiler to the turbine generator.
With prior art gate valves heretofore used for blowdown service, it was necessary to install bypass piping and valve systems to permit a controlled amount of steam to continuously flow downstream in order to maintain the downstream piping temperature substantially constant at all times. Furthermore, gate valves used for this blowdown function were slow to open, and subject to gate and seating damage. As a consequence, every time a blowdown valve would be damaged by flying objects it would be necessary to shut down the line and replace the valve, a very costly and time consuming operation. This resulted in delays of from several days to several weeks before the blowdown operation could be completed. Assuming a conservative cost of service to be 400k per day, every day that a utility plant cannot generate electricity results in a considerable loss to the utility plant.